- #1

skrat

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## Homework Statement

Calculate the flow over the upper surface of sphere ##x^2+y^2+z^2=1## with normal vector pointed away from origin. Vector field is given as ##\vec{F}=(z^2x,\frac{1}{3}y^3+tan(z),x^2z+y^2)##

## Homework Equations

Gaussian law: ##\int \int _{\partial \Sigma }\vec{F}d\vec{S}=\int \int \int _{\Sigma }\nabla \vec{F}dV##

## The Attempt at a Solution

The idea here is to see that ##\int \int _{\partial \Sigma }\vec{F}d\vec{S}+\int\int_O \vec{F}d\vec{S}=\int \int \int _{\Sigma }\nabla \vec{F}dV##

Where I marked the circle (plane) at ##z=0##.

Well, I know how to solve this problem yet I have some problems, let me show you where... Let's start with the integral on the right side. We will need spherical coordinates ##x=rsin\theta cos\phi ##, ##y=rsin\theta sin\phi ## and ##z=rcos\phi ##. The integral is than:

##\int \int \int _{\Sigma }\nabla \vec{F}dV)=\int_{0}^{2\pi }d\phi \int_{0}^{\pi /2}d\theta\int_{0}^{1}(r^2sin^2\theta cos^2\phi +r^2sin^2\theta sin^2\phi +r^2cos\theta )r^2sin\theta dr##

one ##r^2## comes from Jacobian determinant.

Let's forget about all the ##\theta ## and ##\phi ##. Integration by r is obviously ##\frac{r^5}{5}##.

OK.

Now the left side of equation ##\int \int _{\partial \Sigma }\vec{F}d\vec{S}+\int\int_O \vec{F}d\vec{S}=\int \int \int _{\Sigma }\nabla \vec{F}dV##, specifically integral over surface ##O##.

Using polar coordinates ##x=rcos\phi ##, ##y=rsin\phi ## and obviously ##z=0##. The parameterization is than ##r(r,\phi )=(rcos\phi ,rsin\phi , 0)## and ##r_r \times r_{\phi}=(0,0,r)##. Therefore the integral over ##O## for ##z=0##:

##\int\int_O \vec{F}d\vec{S}=-\int_{0}^{2\pi }d\phi \int_{0}^{1}(0,something,r^2sin^2 \phi)(0,0,r)dr##

Jacobian is included in normal vector. Sorry, for using "something" instead of calculating it.

Now let's again forget about all the angles and integrate by ##r##, the integral is than ##\frac{r^4}{4}##..

My question here: WHAT IS WRONG? The units don't match!

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